Rotating stall is an aerodynamic instability which determines the maximum pressure rise capabilities of a turbo-compressor. At that maximum any further demand will lead to the formation of a small patch (usually referred to as a "cell") of disturbed flow in the blading which can then spread rapidly to engulf a large proportion of the annular cross-section of the compressor. As the stall cell forms fully, the delivery pressure at the exit from the compressor falls off just as rapidly. This type of disturbance is called rotating stall because the disturbed or "blocked" sector of the annulus will rotate with the blading, at roughly half (typically between 0.8 and 0.3) the speed of the blading.
The system-related instability of surge occurs when a compressor is coupled to a large downstream volume, eg. pipework in an industrial plant or the combustion chamber in a gas turbine. If the pressure rise capability of the compressor is exceeded and a stall condition is initiated there is a fall in delivery pressure which allows compressed gas in the volume downstream of the compressor to blow back through the compressor. In extreme cases this can lead to flames spewing out of the front of an engine. Venting the downstream volume in this way lowers the back pressure on the compressor; the stall condition disappears and the pressure downstream is able to rise again as the downstream volume is refilled. The stalling, venting, refilling cycle will thus start again and a continuous sequence of surge cycles can ensue if the operating conditions remain unchanged.
It has been proposed in GB 1481031 to detect the flow reversal that occurs with surge, and then to bleed off the boundary layer in the downstream region of the compressor to return this air to the inlet. Such a procedure is inherently inefficient, however, not least because its operation relies upon the surge condition becoming established.
Both stall and surge will limit the operating range of a machine and are damaging conditions. Rotating stall leads to very high temperatures in the compressor and surge leads to to violent bending loads on the blading. The accepted way of avoiding these dangers has been to ensure the compressor does not work close to its peak pressure rise. In recent times, however, it has been suggested that active control techniques might be employed to improve performance. Reference can be made to GB 2191606A and to Epstein AH, Ffowcs-Williams JE, and Greitzer EM, "Active Suppression of Aerodynamic Instabilities in Turbomachinery", AIAA Journal of Propulsion and Power, Vol 5, No 2 1989.
In U.S. Pat. No. 4196472 (Ludwig et al) a stall control system for an axial flow compressor is disclosed in which signals from a number of pressure transducers within the compressor are compared with a reference signal, the value of which is related to the operating conditions in the compressor. The signals from the individual transducers are compared sequentially with the reference signal so that corrective action can be initiated when an abnormal value is sensed by any one sensor. The signals are also summed, and the strength of the corrective action is determined by the summed signal value. The form of corrective action described in U.S. Pat. No. 4196472 takes the form of a controllable bleed from the compressor gas passage, and control of the stagger angle of the stator blades is also suggested.